A frog that can perch on the tip of your finger with room to spare has been claimed as the world's smallest vertebrate species, smaller still than a fish that got the title in 2006.

But the discoverer of another fish disputes the claim.

An article in the journal PLoS One named Paedophryne amauensis as the world's smallest animal with a spine.

The adult frogs are about three-tenths of an inch long, and a millimetre or so smaller than a carp found on the Indonesian island of Sumatra.

The frogs are so small that Louisiana State University herpetologist and environmental biologist Christopher Austin had to enlarge close-up photos to describe them.

But the males of a species of deep-sea anglerfish are about 2mm smaller, said University of Washington ichthyologist Theodore Pietsch, who described them in 2006. The males do not have stomachs and live as parasites on 1.8-inch-long females.

Mr Austin discovered the tiny frogs - along with another small frog species - in August 2009 while on a trip to Papua New Guinea to study the extreme diversity of the island's wildlife. He said he knew about the anglerfish but felt that average species size made more sense for comparison.

Steven J. Beaupre, a University of Arkansas scientist and president-elect of the American Society of Ichthyologists and Herpetologists, said many vertebrates have males and females of very different sizes, "so it is reasonable that the world's smallest vertebrate may end up being either the males or the females of some specific fish or amphibian species".

He said he does not pay attention to "tiniest" reports, but the frogs themselves are a significant discovery. "The discovery of two new frog species comes as great news against the background of more prevalent accounts of tropical amphibian extinction," he wrote in an email.

Knowing about such tiny creatures and their ecology, he said, helps scientists "better understand the advantages and disadvantages of extreme small size and how such extremes evolve. Fundamentally, these tiny vertebrates provide a window on the principles that constrain animal design".